Land vertebrate faunachrons (LVFs) are
biochronological units used to correlate and date terrestrial sediments and fossils based on their
tetrapod
Tetrapods (; ) are four-limb (anatomy), limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant taxon, extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (p ...
faunas.
First formulated on a global scale by
Spencer G. Lucas in 1998, LVFs are primarily used within the
Triassic Period
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
(252 - 201
Ma),
though Lucas later designated LVFs for other periods as well.
Eight worldwide LVFs are defined for the Triassic. The first two of these LVFs, the Lootsbergian and Nonesian, are based on
South African __NOTOC__
South African may relate to:
* The nation of South Africa
* South African Airways
* South African English
* South African people
* Languages of South Africa
* Southern Africa
Southern Africa is the southernmost subregion of the Afric ...
synapsids
Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...
and faunal assemblage zones estimated to correspond to the
Early Triassic
The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which is a ...
. These are followed by the Perovkan and Berdyankian, based on
temnospondyl
Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carb ...
amphibians and
Russian assemblages estimated to be from the
Middle Triassic
In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma ...
. The last four LVFs, the Otischalkian, Adamanian, Revueltian, and Apachean, are based on
aetosaur and
phytosaur reptiles common in the
Late Triassic
The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch ...
of the southwestern United States.
The LVF system, though widely used, is also a controversial application of
biostratigraphy
Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictionary of Bio ...
, as many Triassic tetrapods are rife with complications which endanger their utility as
index fossils
Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictionary of Bio ...
. Limited occurrences, inaccurate age estimates, overlapping LVF faunas, or
taxonomic disagreement may jeopardize global correlations between Triassic tetrapods. This could render some LVFs as misleading assessments of Triassic faunal change through time.
Regardless, Late Triassic phytosaurs are considered to have strong biostratigraphic utility even among detractors of Lucas's system.
Lucas's LVFs
Tetrapod biostratigraphy has been used for the Triassic of
South Africa
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring count ...
since 1906 and
Argentina
Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, t ...
since 1966,
but without much connection to global faunas.
Starting in 1993,
New Mexico Museum of Natural History and Science paleontologist
Spencer G. Lucas and his colleagues began to define tetrapod biostratigraphy intervals in the Triassic of
China and eastern and western
North America. These named biostratigraphic intervals were inspired by the
Land Mammal Age (LMA) system already in use for
Cenozoic
The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configu ...
faunal assemblages.
Triassic tetrapod biozones, under the term "land vertebrate faunachrons" (LVFs) were formalized on a global level by Lucas in 1998. They were diagnosed by a primary
index fossil (a particular genus of widespread time-constrained tetrapod) and characterized by a faunal
type
Type may refer to:
Science and technology Computing
* Typing, producing text via a keyboard, typewriter, etc.
* Data type, collection of values used for computations.
* File type
* TYPE (DOS command), a command to display contents of a file.
* Ty ...
assemblage (distinguishing collection of taxa) from a fossiliferous
geological formation
A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics (lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock expo ...
. Together, the defining index fossil and assemblage could be used to correlate fossil assemblages worldwide.
Updates to this system have been published continuously for Triassic LVFs, which remain a heavily-discussed topic in the study of Triassic chronology.
Lucas has also defined LVFs for the
Permian
The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Pale ...
,
Jurassic
The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...
, and
Carboniferous, though these are not as widely used as his Triassic LVFs.
Later authors characterized Lucas's LVFs as "
interval eubiochrons". This means that they correspond to a segment of time (and strata) between two paleobiological events: the
first appearance datum
First appearance datum (FAD) is a term used by geologists and paleontologists to designate the first appearance of a species in the geologic record. FADs are determined by identifying the geologically oldest fossil discovered, to date, of a part ...
(FAD) of one index taxon and the FAD of another.
A first appearance datum is a point in the geological record with the earliest known fossil of a given animal, which can estimate when that animal
speciates or evolves into existence. As an example, the Lootsbergian LVF is defined as the period of time between the FAD (estimated speciation) of ''
Lystrosaurus'' and the FAD (estimated speciation) of ''
Cynognathus''. Some taxa which are index fossils for one stage may persist into a later stage.
List of Triassic LVFs
Criticism
Several paleontologists have independently questioned the validity of Lucas’s system, criticizing its inconsistent and often contradictory approach to taxonomy and faunal correlations.
Endemic index taxa
Many index taxa are very rare or endemic to a single continent, and have no relevance to a global biostratigraphy system. These include ''
Doswellia'', ''
Longosuchus'', ''
Typothorax
''Typothorax'' is an extinct genus of typothoracine aetosaur that lived in the Late Triassic. Its remains have been found in North America. Two species are known: ''T. coccinarum'', the type species, and ''T. antiquum''.
Description
''Typothor ...
'', “''
Pseudopalatus''” (''
Machaeroprosopus''), ''
Redondasaurus
''Redondasaurus'' is an extinct genus of phytosaur from the Late Triassic (221.5 to 201.6 million years ago) of the southwestern United States. It was named by Hunt & Lucas in 1993, and contains two species, ''R. gregorii'' and ''R. bermani''. I ...
'', and ''
Redondasuchus
''Redondasuchus'' is an extinct genus of aetosaur. It may be a junior synonym of '' Typothorax coccinarum'', another aetosaur. ''Redondasuchus'' is a member of the clade Typothoracisinae within the subfamily Aetosaurinae, and lived during the mi ...
'', among others.
For the Berdyankian LVF, very few species are shared between the index assemblage (the
Bukobay Formation of Russia) and other correlated assemblages. Direct relationships between Russian, German, and South American
dicynodonts
Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typica ...
are conjectural and based on undiagnostic European fragments.
Imprecise or inaccurate time scales
Index taxa for a given LVF often range into the succeeding LVF, blurring the distinction between the two time periods. ''
Angistorhinus
''Angistorhinus'' (meaning "narrow snout" or "hook snout") is an extinct genus of phytosaur known from the Late Triassic period of Texas and Wyoming, United States. It was first named by Mehl in 1913 and the type species is ''Angistorhinus grand ...
'', ''
Hyperodapedon'', ''
Paleorhinus
''Paleorhinus'' (Greek: ''"Old Nose"'') is an extinct genus of widespread basal phytosaur known from the Late Triassic (late Carnian stage). The genus was named in 1904 based on the type species ''Paleorhinus bransoni'', which is known from Wyom ...
''/''
Parasuchus
''Parasuchus'' is an extinct genus of basal phytosaur known from the Late Triassic (late Carnian to early Norian stage) of Andhra Pradesh and Madhya Pradesh, India. At its most restricted definition, ''Parasuchus'' contains a single species, ' ...
'' (all Otischalkian index taxa) range into the Adamanian, fossils referred to ''
Rutiodon'' (an Adamanian index taxon) range into the Revueltian, and ''
Metoposaurus
''Metoposaurus'' meaning "front lizard" is an extinct genus of stereospondyl temnospondyl amphibian, known from the Late Triassic of Germany, Italy, Poland, and Portugal. https://www.app.pan.pl/archive/published/app47/app47-535.pdf This mostly ...
'' can be found throughout the Otischalkian, Apachian, and Revueltian LVFs.
Lucas's approach to correlating LVFs with global marine stages has been met with criticism. The Triassic timescale is under constant revision from a series of age dating methods, including
magnetostratigraphy
Magnetostratigraphy is a geophysical correlation technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout the section. The samples are analyzed to determine their ' ...
,
cyclostratigraphy
Cyclostratigraphy is a subdiscipline of stratigraphy that studies astronomically forced climate cycles within sedimentary successions.
Orbital changes
Astronomical cycles (also known as Milankovitch cycles) are variations of the Earth's or ...
,
radiometric dating
Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares t ...
, and biozones of marine invertebrates such as
conodonts and
ammonoids.
However, there are only a few areas where fossils of Triassic land tetrapods and marine organisms overlap, mostly restricted to coastal sediments in central Europe.
Palynomorph and
conchostracan biozones can help correlate terrestrial strata to an extent. One complication is that Lucas's view of the Late Triassic time scale contradicts the consensus established by other biostratigraphers. Most paleontologists estimate that the three stages of the Late Triassic (Carnian, Norian, and Revueltian) are strongly unequal in size, with the Norian far longer than the Carnian. Under this consensus "long-Norian" hypothesis, the Carnian-Norian boundary is close to 228 Ma. Lucas, on the other hand, prefers a "short-Norian" perspective, with a lengthier Carnian stage and a Carnian-Norian boundary at around 220 Ma.
For example, Lucas has maintained that the lower part of the Chinle Formation (the
Blue Mesa Member and equivalent units) is Carnian (>220 Ma) in age. This was justified by the assumption that fossils of ''
Stagonolepis'', a European aetosaur, can be found in North and South America, allowing correlation between these regions. However, this proposed widespread occurrence of ''Stagonolepis'' is a debatable, as many species assigned to the genus may not be closely related (see below).
According to the "short-Norian" interpretation, these lower Chinle Formation, and other strata of the Adamanian LVF, would be firmly pre-Norian in age, suggesting that any taxonomic change between the Adamanian and Revueltian represents a Carnian-Norian extinction event. However, the consensus "long-Norian" interpretation firmly places Adamanian strata of North America into the Norian stage (<228 Ma). The Norian age of the lower Chinle Formation has been independently confirmed by
U-Pb dating and magnetostratigraphic correlations to global time scales.
Conversely, other "Adamanian" strata, such as fossiliferous layers in the lower
Ischigualasto Formation of Argentina, can be assigned to the late Carnian (~231 Ma).
This supports the conclusion that LVFs such as the Adamanian are fraught with uncertain time estimates brought on by weak correlations on a global scale.
Some authors have elected to ignore LVFs in favor of older and more localized biostratigraphic units. Named tetrapod assemblages zones (AZs) were well-established for the Triassic of Gondwana prior to the LVF, and recent updates have helped to constrain these units with greater clarity and agreement than global correlations. In Argentina,
Bonaparte (1966) established the Chanarian (named after the
Chañares Formation) and the Ischigualastian (named after the Ischigualasto Formation). Equivalents faunas are easily traced across Brazil, Africa, and India. These two biostratigraphic zones correlate with Lucas's Berdyankian, Otischalkian, and Apachean LVFs, but do not precisely overlap in time with those LVFs. Moreover, aetosaurs and phytosaurs, which are common in the Northern Hemisphere, are rarer and more scattered in the Southern Hemisphere. As a result, Gondwanan assemblage zones are defined by more common Southern taxa. For example, the Ischigualastian zone is defined by the
rhynchosaur ''
Hyperodapedon'' and the
cynodont
The cynodonts () ( clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variet ...
''
Exaeretodon,'' as well as the aetosaur ''
Aetosauroides'' and
herrerasaurid
Herrerasauridae is a family of carnivorous dinosaurs, possibly basal to either theropods or even all of saurischians, or even their own branching from dracohors, separate from dinosauria altogether. They are among the oldest known dinosaurs, fi ...
dinosaurs
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
.
Taxonomic uncertainty and dubious correlations

Some correlations are based on connections between fragmentary or poorly-constrained taxa rather than direct correlations between type assemblages or LVF-defining index taxa. For example, the
Ermaying Formation of China is correlated with the
Moenkopi Formation
The Moenkopi Formation is a geological formation that is spread across the U.S. states of New Mexico, northern Arizona, Nevada, southeastern California, eastern Utah and western Colorado. This unit is considered to be a group in Arizona. Par ...
of the United States via a tenuous (and likely unjustifiable) comparison between proposed
erythrosuchid
Erythrosuchidae (meaning "red crocodiles" in Greek) are a family of large basal archosauriform carnivores that lived from the later Early Triassic (Olenekian) to the early Middle Triassic (Anisian).
Naming
The family Erythrosuchidae was named b ...
fossils. The primary index fossil of the Perovkan LVF, ''
Eocyclotosaurus'', is absent from China.
One particularly contradictory index fossil is ''
Mastodonsaurus'', the defining index fossil of the Berdyankian LVF. Fossils referable to this genus can be found across
Ladinian
The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between Ma and ~237 Ma (million years ago). The Ladinian was preceded by the Anisian and succeeded by the Carnian (part of the Upper or Late Triassic) ...
-age Europe, but the proposed Russian species (''M. torvus'') may be unrelated to the endemic German
type species
In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen( ...
(''M. giganteus''). Moreover, if one approaches ''Mastodonsaurus'' from a broader taxonomic perspective (as expected if ''M. torvus'' is included), they must also incorporate
Anisian
In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from million years ago until million years ago. The Anisian Age succeeds the Olenekian Age (part of the Lower Trias ...
and
Carnian
The Carnian (less commonly, Karnian) is the lowermost stage of the Upper Triassic Series (or earliest age of the Late Triassic Epoch). It lasted from 237 to 227 million years ago (Ma). The Carnian is preceded by the Ladinian and is followe ...
material referred to the genus, including the small species ''
“Heptasaurus” cappelensis''. This precludes any reason to use ''Mastodonsaurus'' as a time-constrained index taxon.
Some LVFs are based on
evolutionary grades as index taxa. This ignores the potential for high diversity and long temporal ranges within a given grade, and may lead to arbitrary and subjective inclusion or exclusion of descendant taxa. “''
Stagonolepis''”, in its broadest form, is a
wastebasket taxon
Wastebasket taxon (also called a wastebin taxon, dustbin taxon or catch-all taxon) is a term used by some taxonomists to refer to a taxon that has the sole purpose of classifying organisms that do not fit anywhere else. They are typically define ...
of
basal
Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''.
Science
* Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure
* Basal (medicine), a minimal level that is nec ...
aetosaurs ranging through the Otischalkian and Apachean. Lucas’s usage of ''Stagonolepis''
lumps in many genera separated by other authors, such as ''
Aetosauroides'' and ''
Calyptosuchus''.
A similar situation occurs in ''Paleorhinus/Parasuchus'', which has historically been used as a persistent grade of early phytosaurs. On the other hand, the characteristic phytosaur (''Redondasaurus'') and aetosaur (''Redondasaurus'') genera of the Apachean LVF are very similar to, and perhaps synonymous with, index taxa of the underlying Revueltian LVF: “''Pseudopalatus''” (''Machaeroprosopus'') and ''Typothorax'', respectively.
Martz & Parker (2017) revision
Although the utility of a global LVF system is questionable, LVF-derived biostratigraphy may be useful in limited circumstances. Phytosaurs in particular have played a large role in the tetrapod biostratigraphy of the
Chinle and
Dockum Group
The Dockum is a Late Triassic (approximately late Carnian through Rhaetian, or 223–200 Ma) geologic group found primarily on the Llano Estacado of western Texas and eastern New Mexico with minor exposures in southwestern Kansas, eastern Color ...
of the southwest United States. A revision of the LVF system in this narrow context was undertaken by
Jeff Martz
Jeff is a masculine name, often a short form ( hypocorism) of the English given name Jefferson or Jeffrey, which comes from a medieval variant of Geoffrey.
Music
* DJ Jazzy Jeff, American DJ/turntablist record producer Jeffrey Allen Townes ...
and
Bill Parker (2017), retaining several names and concepts previously used by Lucas and colleagues.
Martz and Parker argued that the term "faunachron" was misleading and redundant, as each "faunachron" is bound by a single taxon rather than an assemblage (
fauna
Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as ''biota''. Zoo ...
) of multiple taxa. They preferred using a specific type of interval
biozone
In biostratigraphy, biostratigraphic units or biozones are intervals of geological strata that are defined on the basis of their characteristic fossil taxa, as opposed to a lithostratigraphic unit which is defined by the lithological properties ...
known as a
teilzone, referring to a local interval of strata equivalent to an interval of time.
The base of each teilzone was marked by the Lowest known Occurrence (LOk) of a particular category of phytosaur, i.e. the oldest layer where fossils of that category are found in the study area. LOks are local points in time and stratigraphy, disregarding occurrences in other regions or the estimated time of
speciation
Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution withi ...
. For the Otischalkian, Adamanian, and Revueltian, the top of each teilzone is marked by the LOk of a more exclusive subgroup of phytosaurs. The top of the Apachean is marked by the LOk of ''
Protosuchus
''Protosuchus'' is an extinct genus of carnivorous crocodylomorph from the Early Jurassic. The name ''Protosuchus'' means "first crocodile", and is among the earliest animals that resemble crocodilians. ''Protosuchus'' was about in length and ...
'', an
Early Jurassic
The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 201.3 Ma  ...
crocodylomorph
Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction.
During Mesozoic and early Cenozoic times, cro ...
, as with Lucas's system.
"Faunachrons" could also be defined beyond the constraints of teilzones; other biozonation categories include holochronozones (a stratigraphic interval, involving multiple study areas) and holochrons (an estimated time interval, involving the time of speciation or immigration into the region). Each phytosaur-based "faunachron" could be considered a teilzone (in local biostratigraphy), an estimated holochronozone (in regional
chronostratigraphy
Chronostratigraphy is the branch of stratigraphy that studies the ages of rock strata in relation to time.
The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geologica ...
), or an estimated holochron (in regional
biochronology).
One complication in defining biozones based on phytosaurs is instability in phytosaur systematics. Many proposed phytosaur taxa are dubious,
paraphyletic
In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
(such as ''
Leptosuchus'' and ''
Machaeroprosopus)'' or have unclear relationships to each other. Nevertheless, a series of nested
clades
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term ...
is apparent in most recent overviews. Rather than relying on a single index taxon per biozone, Martz and Parker allowed multiple representatives per a given stage of phytosaur evolution. These representatives were chosen based on their occurrence in the southwest United States, commonness, and relatively stable phylogenetic position despite paraphyly in some circumstances.
Adamanian-Revueltian turnover
Although most LVFs or equivalent concepts are not marked by major biotic changes, one exception is apparent in the southwest United States. The boundary between the Adamanian and Revueltian zones is marked by a faunal turnover, an event where several tetrapod species quickly disappear from the fossil record as others appear for the first time. At
Petrified Forest National Park
Petrified Forest National Park is an American national park in Navajo and Apache counties in northeastern Arizona. Named for its large deposits of petrified wood, the park covers about , encompassing semi-desert shrub steppe as well as highly ero ...
, the event occurs in the Jim Camp Wash beds. This sediment layer is positioned in the middle of the Chinle Formation's
Sonsela Member, and would have been deposited around 215 million years ago. ''
Trilophosaurus'', ''
Poposaurus'', ''
Desmatosuchus'',
dicynodonts
Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typica ...
, and non-
mystriosuchin phytosaurs are
extirpated
Local extinction, also known as extirpation, refers to a species (or other taxon) of plant or animal that ceases to exist in a chosen geographic area of study, though it still exists elsewhere. Local extinctions are contrasted with global extinct ...
from the area around this time, while
metoposaurs
Metoposauridae is an extinct family of trematosaurian temnospondyls. The family is known from the Triassic period. Most members are large, approximately long and could reach 3 m long.Brusatte, S. L., Butler R. J., Mateus O., & Steyer S. J. (201 ...
and
allokotosaurs
Allokotosauria is a clade of early archosauromorph reptiles from the Middle Triassic, Middle to Late Triassic known from Asia, Africa, North America and Europe. Allokotosauria was first described and named when a new monophyletic grouping of spec ...
as a whole decline in abundance. New species of aetosaurs and phytosaurs replaced losses across the purported boundary event.
Palynomorph assemblages overturn to more dry adapted species, and a higher concentration of
pedogenic
Soil formation, also known as pedogenesis, is the process of soil genesis as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order (anisotropy) within soils. These alterations l ...
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonat ...
nodules may also support increasing aridity.
The cause and relevance of this turnover is debatable, as it may indicate only a small localized extinction. The
Manicouagan Impact, the second-largest bolide impact of the
Mesozoic Era
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...
(besides the
Chicxulub Impact which caused the
K-Pg Mass Extinction at 66 Ma), is dated to around 215.4 Ma. While certainly large enough to momentarily devastate areas near the impact point in
Quebec
Quebec ( ; )According to the Government of Canada, Canadian government, ''Québec'' (with the acute accent) is the official name in Canadian French and ''Quebec'' (without the accent) is the province's official name in Canadian English is ...
, broader environmental effects of the Manicouagan impact are mostly conjectural.
Besides the Adamanian-Revueltian turnover, the impact has also been linked to a minor marine extinction in eastern
Panthalassa
Panthalassa, also known as the Panthalassic Ocean or Panthalassan Ocean (from Greek "all" and "sea"), was the superocean that surrounded the supercontinent Pangaea, the latest in a series of supercontinents in the history of Earth. During th ...
.
Alternatively, the Adamanian-Revueltian turnover may be a consequence of the gradual aridification of western Pangea as it drifted north into arid latitudes.
Comparative estimates of extinction rates and occurrences find little support for a synchronized Adamanian-Revueltian turnover, and instead support a model where extinctions are stretched out over several million years. For most species, extinction probabilities are "decoupled" in time from other species, as well as geological or climatological drivers. The only plausible correlation is between the Manicouagan Impact and palynomorph turnover, and even then the probability of synchronicity is only about 34%.
References
{{reflist
Biochronology
Regional geologic time scales
Triassic geochronology
Triassic life